1. Field of the Invention
The invention relates to an image forming apparatus, such as a copier, a facsimile, a printer, or the like, provided with a development unit that agitates and conveys a toner inside the unit and then supplies it to a developer roller, and more specifically to a drive control method of the development unit.
2. Description of Related Art
In recent years, as image carriers for an image forming apparatus using an electrophotographic process, amorphous silicon (hereinafter, referred to as a-Si) photoconductive drums have been widely used. The a-Si photoconductive drum is an excellent image carrier with a high degree of hardness, excellent durability, a capability of maintaining high image quality with little deterioration in its properties as a photoconductor even after its long-term use, which requires low running costs, is easy to handle, and provides a high level of environmental safety.
Such an image forming apparatus using an a-Si photoconductive drum is known to have a property that easily causes image blurring. That is, charging the photoconductive drum by using a charge unit results in ozone generation caused by discharge from the charge unit. This ozone decomposes a component in the air, whereby an ion product, such as NOx, SOx, or the like, is generated. Due to its water-soluble nature, this ion product adheres to the photoconductive drum and enters into a structure formed, with a roughness of approximately 0.1 μm, on the photoconductive drum surface, so that the ion product cannot be removed by a cleaning system used in a general purpose machine. Further, this ion product intakes moisture contained in the air, thereby causing a reduction in the resistance of the photoconductive drum surface. As a result, lateral charge flow occurs at an edge part of an electrostatic latent image formed on the photoconductive drum surface, thus causing image blurring.
FIG. 6 is a schematic diagram showing the configuration of an image formation part of a conventional image forming apparatus. In FIG. 6, disposed along the rotation direction of a photoconductive drum 41 (an arrow A direction) in the image formation part 32 are: a charge part 42, an image writing part (laser scan unit) 43, a development unit 21, a transfer part 45, a cleaning part 46, and an electricity removing device 48.
The photoconductive drum 41 is, for example, formed by laying a photosensitive layer of a-Si on an aluminum drum, and is configured to have its surface charged by the charge part 42. Then, on the surface receiving a laser beam from the image writing part 43 to be described later, an electrostatic latent image is formed which is charged attenuatedly. The charge part (charger) 42 charges the surface of the photoconductive drum 41 through its discharge (for example, corona discharge) which is achieved by, for example, being supplied with a high voltage using a thin wire or the like as an electrode.
The image writing part (LSU) 43, based on image data, irradiates the photoconductive drum 41 with an optical beam (for example, laser beam) to thereby form an electrostatic latent image on the surface of the photoconductive drum 41. The development unit 21 is provided with a developer roller 21a that is so arranged as to oppose the photoconductive drum 41, and, by using a developer roller 21a, has a toner stored therein adhere to the electrostatic latent image on the photoconductive drum 41 to thereby form a toner image. The toner stored in the development unit 21 is, for example, a two-component toner, composed of a toner component and a carrier, and a one-toner component toner composed of a toner component only.
The cleaning part 46 removes a toner remaining on the surface of the photoconductive drum 41 (residual toner) after a toner image is shifted (transferred) to a sheet, and is composed of, for example, a rubbing roller 11 which is brought by a spring 7 into line contact with the photoconductive drum 41 in the longitudinal direction thereof, a cleaning blade 12, and the like. The electricity removing device 48 exposes the surface of the photoconductive drum 41 to, for example, an LED or the like to thereby remove the surface potential after the toner image is transferred.
As already known, after the removal of electricity by the electricity removing device 48, the electrostatic latent image is recorded by the image writing part 43 onto the photoconductive drum 41 uniformly charged by the charge part 42, then this electrostatic latent image is transformed into a visible toner image by the development unit 21 through reversal development, and then the toner image is transferred onto a sheet 10 by the transfer part 45. The toner not transferred by the transfer part 45 is removed as a residual toner from the surface of the photoconductive drum 41 by the cleaning part 46, and the removed residual toner is transferred by a toner collector, such as a collecting screw 13 or the like, to a waste bottle, not shown.
In the photoconductive drum 41, a heater 14 is arranged. Through electricity distribution to this heater 14, an energy that separates the moisture taken in by the ion product is provided, thereby suppressing a reduction in the resistance of the photoconductive drum 41 surface under high-humidity environments.
FIG. 7 is an enlarged sectional view of a conventional development unit, with the photoconductive drum 41 flipped horizontally from the one shown in FIG. 6. The development unit 21 is configured to include a casing 5, a cover 6, a first agitation screw 7, a second agitation screw 8, the developer roller 21a, and a control blade 22. The casing 5 stores a toner, and is formed by partitioning, by the partition plate 9 integrated therewith, a first storage chamber 15 and a second storage chamber 16. In this first storage chamber 15, the first agitation screw 7 is disposed, while the second agitation screw 8 is disposed in the second storage chamber 16.
The first agitation screw 7 conveys a toner or the like stored in the first storage chamber 15 while agitating it and leads it to the second storage chamber 16. The second agitation screw 8 conveys the toner or the like conveyed to the second storage chamber 16 while agitating it and supplies it to the developer roller 21a. In both end parts of the casing 5 in the longitudinal direction thereof (the paper surface direction in the figure), the partition plate 9 does not exist, permitting toner reception and delivery between the first agitation screw 7 and the second agitation screw 8. The first agitation screw 7 and the second agitation screw 8 are configured to have helical blades 7b and 8b provided around their respective centers, i.e., spindles 7a and 8a, and are rotatably supported in the casing 5 in parallel to each other.
Inside the developer roller 21a, a magnetic field generating member 23 is Fixed, which has six magnetic poles 23a to 23f composed of N poles 23a, 23c, and 23e and S poles 23b, 23d, and 23f. The N pole 23a of the magnetic field generating member 23 opposes the control blade 22; thus, the use of a non-magnetic body or a magnetic body of an S pole as the control blade 22 generates a magnetic field in the direction (direction of an arrow C) attracted to a control part 24.
This magnetic field causes the toner to rise in a brush like form between the control blade 22 and the developer roller 21a. Then, rotation of the developer roller 21a in the direction of an arrow B generates a force acting so as to separate the toner rising in the brush-like form whereby a thin toner layer is formed on the surface of the developer roller 21a. When this thin toner layer moves to the position opposing the photoconductive drum 41, a potential difference between the voltage applied to the developer roller 21a and the surface potential of the photoconductive drum 41 causes a toner image to be formed onto the surface of the photoconductive drum 41.
Further rotation of the developer roller 21a in the direction of the arrow B provides a magnetic field to be attracted by the N pole 23c, so that the toner not used for the toner image formation is collected in the development unit 21. Then, after agitated and conveyed by the second agitation screw 8, due to the magnetic fields of the N pole 23e and S pole 23f the toner adheres again onto the developer roller 21a. That is, not only the gap at the control part 24 but also the magnetic field generated at the control part 24 strictly control the thin toner layer on the developer roller 21a. In addition, provided at the both axially left and right end parts of the developer roller 21a are magnetic seal members (not shown) for preventing the toner held on the developer roller 21a surface from leaking outside.
In such a development unit 21, the developer roller 21a is so arranged as to oppose, in close proximity to, the photoconductive drum 41. Thus, when the photoconductive drum 41 is hest by using the heater 14 (see FIG. 6) during non-image formation where the developer roller 21a is in a resting state, radiant heat from the photoconductive drum 41 causes only the portion of the developer roller 21a opposing the photoconductive drum 41 to become locally high in temperature.
Thus, as shown in FIG. 8, the side of the developer roller 21a opposing the photoconductive drum 41 thermally expands whereby the developer roller 21a is bent axially (indicated by a solid line of FIG. 8 ). As a result, a distance d between the photoconductive drum 41 and the developer roller 21a periodically fluctuates when the developer roller 21a rotates, thus causing, in particular, periodical image unevenness, such as image fogging on a white paper part, concentration unevenness on a grey image, or the like.
Thus, a method has been proposed which prevents the deformation of the developer roller when a power is distributed to the heater during the non-image formation period. Patent publication 1, for example, discloses a development unit and an image forming apparatus provided with a structure not having a hollow in at least part of the developer roller by stuffing a filling material having a higher heat conductivity than the developer roller. However, the method of patent publication 1 suffers from problems of more complicated structure of the developer roller, a larger number of components used, and higher manufacturing costs of the developer roller and a development unit using this developer roller.
Moreover, there is a possible method in which, as during a image formation period, the developer roller is rotated even during the non-image formation period so as to prevent the developer roller from becoming locally high in temperature. However, unnecessarily driving the development unit results in an increased mechanical stress, thereby promoting toner deterioration in the development unit. Patent publication 2 discloses a method of controlling a heater provided in an image forming apparatus in accordance with the operation mode of each part of the apparatus. However, the method of patent publication 2 does not control the operation of each part of the apparatus in accordance with the operation of the heater, but controls power supply to the heater in accordance with the operation of each part of the apparatus for the purpose of reducing the peak power consumption by the entire apparatus, and thus is not applicable for preventing the deformation of the developer roller.
[Patent Publication 1] JP-A-H11-174820
[Patent Publication 2] JP-A-2002-40887